Energies 2010, 3, 866-885; doi:10.3390/en3040866 OPEN ACCESS
energies ISSN 1996-1073 www.mdpi.com/journal/energies Review
Surface-Modified Membrane as A Separator for Lithium-Ion Polymer Battery Jun Young Kim 1,2,* and Dae Young Lim 3,* 1
2
3
Corporate R&D Center, Samsung SDI Co. Ltd., 428-5 Gongse-dong, Giheung-gu, Yongin-si, Gyeonggi-do, 446-577, Korea Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Fusion Textile Technology Team, Korea Institute of Industrial Technology, 1271-18 Sa-1 dong, Sangrok-gu, Ansan-si, Gyeonggi-do, 426-173, Korea
* Authors to whom correspondence should be addressed; E-Mails:
[email protected] (J.Y.K);
[email protected] (D.Y.L); Tel.: +82-31-8040-6231; Fax: +82-31-8040-6220. Received: 26 January 2010; in revised form: 21 February 2010 / Accepted: 26 February 2010 / Published: 23 April 2010
Abstract: This paper describes the fabrication of novel modified polyethylene (PE) membranes using plasma technology to create high-performance and cost-effective separator membranes for practical applications in lithium-ion polymer batteries. The modified PE membrane via plasma modification process plays a critical role in improving wettability and electrolyte retention, interfacial adhesion between separators and electrodes, and cycle performance of lithium-ion polymer batteries. This paper suggests that the performance of lithium-ion polymer batteries can be greatly enhanced by the plasma modification of commercial separators with proper functional materials for targeted application. Keywords: battery; membrane; plasma; separator; surface modification
1. Introduction As there is a growing demand for high-performance rechargeable batteries used in portable electronic equipment, mobile products, and communication devices, the lithium-based batteries as a power source are of great scientific interest. Among many types of rechargeable batteries, the
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lithium-ion polymer battery (LIPB) has potential to be used in a broad range of industries, because it can be produced in a variety of forms, thus making it possible to fabricate readily portable batteries in the required shapes for various electronic applications [1]. In LIPB, the separator placed between the cathode and the anode is one of critical components. Its primary function is to effectively transport ionic charge carriers between two electrodes as an efficient ionic conductor as well as to prevent the electric contact between them as a good electric insulator [1–3]. The separators must be chemically or electrochemically stable and have mechanical strength sufficiently enough to sustain the battery-assembly processes because the separator has a significant effect on the manufacturing process and the performance of LIPB [3–5]. Commercially available polyolefin separators have good mechanical and thermal properties and effectively prevent thermal runaway caused by the electrical short-circuits or rapid overcharging. However, these separators do not readily absorb the electrolyte solvents with high dielectric constants, such as ethylene carbonate (EC), propylene carbonate (PC), and -butyrolactone (GBL), because of their hydrophobic surfaces with low surface energy, and they have poor ability to retain the electrolyte solutions [6,7]. In addition, the solvent leakage from the interfaces between the electrodes or the opposite sides of current collectors often causes the deterioration of the cycle life of LIPB [8]. To overcome these drawbacks of conventional polyolefin separators, much research has been undertaken to develop alternative separators that are compatible with polar liquid electrolytes and stable with electrode materials [9–12]. A number of efforts have been made to achieve high-performance polyolefin separators by coating them with the gel polymer electrolyte (GPE) to improve compatibility with various electrolyte solutions as well as the electrochemical properties of LIPB [13–15]. Although these surface-modified polyolefin separators exhibit good mechanical and thermal properties as well as the degree of compatibility with electrolyte solutions, they still have several disadvantages, such as complex multi-step processes and relatively expensive modification of the surface of hydrophobic polyolefin separators with adequate hydrophilic monomers to increase the surface energy enough to absorb the electrolyte solutions. Among the numerous methods of surface modification of polyolefin separators, the radiation process is one of the most promising methods due to the rapid formation of active sites for initiating the reaction through the polymer matrix and the uniformity of polymers over the entire specimen [16]. The plasma process is a preferred and convenient technique when considering a large scale production or commercialization of the membrane. However, to date, studies on the surface modification of polyolefin separators using the plasma technology have rarely been investigated. This paper focuses on the enhancement of the cycle performance of LIPB with the benefit of plasma technology. We begin with a brief summary of the research activities of the surface modification of polyolefin membranes and their structural and physical properties. Subsequent sections discuss the fabrication of novel modified PE membranes by means of the plasma-induced coating process. We expect that this study will help in preliminary evaluation and understanding of the plasma-modified PE membrane as a separator for LIPB and its possible realization. This paper suggests that the modified PE membrane via plasma treatment holds great potential to be utilized as a high-performance cost-effective separator for LIPB. Finally, we conclude this review with personal perspectives on future
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directions in the fundamental research as well as potential applications using plasma-induced coating technology. 2. Fabrication of Separator for LIPB 2.1. General Features The separator is a critical component in LIPB, and its primary function is to facilitate ionic transport between the electrodes as well as to prevent the electric contact of two electrodes. However, the presence of the separator in LIPB induced the electrical resistance and limited space inside the battery to satisfy the need for slimming and safety, which significantly influences the battery performance. Therefore, the fabrication of high-performance separators plays an important role in controlling the overall performance of LIPB, including high power or energy density, long cycle life, and excellent safety. A number of factors influencing the performance of LIPB must be considered in achieving high-performance separators for the battery applications. Among a wide variety of properties for the separators, the following criteria are required to fabricate the separators for LIPB [1–5]: (a) electronic insulator, (b) minimal electric resistance (
